Continuous analyzer for acid content



Nov. 24, 1970 J. J. MORROW 3,542,518

CONTINUOUS ANALYZER FOR ACID CONTENT Filed Aug. 5, 1968 2 Sheets-Sheet 1%HCL i0 ADDED HCL FOUND BY IODINE EQUIVALENTS :DECHLORlNATOR DILUTIONIBODc/min 23 WATER} V V ANAC-HLOR I2 GENERATED MICRO-FLOW MICRO-FLOWPUMP PUMP m 20cc/min PROCESS STREAM 22- l5-3O mg Cu/l FIG.3

SOLUTION l g/Cc INVENTOR. JAMES J. MORROW ATTORNEY Nov. 24, 1970 J. J.MORROW CONTINUOUS ANALYZER FOR ACID CONTENT 2 Sheets-Sheet 2 Filed Au 5,1968 moGz NOE E EMXZZ @004 O mww awwodu INVENTOR JAMES J. MORROWATTORNEY United States Patent US. Cl. 23-253 12 Claims ABSTRACT OF THEDISCLOSURE A system for determining the concentration of a solubleconstituent in an aqueous solution, which constituent is capable ofliberating a halogen when the solution is reacted with a reagentincluding a halide and a halate salt. The system includes anamperometric device responsive to halogens to generate a current inproportion to the concentration of the applied halogen.

RELATED APPLICATION This application is a continuation-in-part of mypending application, Ser. No. 603,504, filed Dec. 21, 1966, nowabandoned.

This invention relates generally to techniques for determining the acidcontent of a pickling bath, and more particularly to a system includingan acid analyzer for continuously monitoring and controlling the acidconcentration of pickling baths for iron and steel.

Pickling is the process for removing surface oxides from metals bychemical and electrochemical reaction. Removal of scale and rust fromiron and steel is accomplished ordinarily by the use of sulfuric orhydrochloric acid. At room temperature, the action of hydrochloric acidis more rapid and is therefore preferable to sulfuric acid for lightoxide films, such as are found in polished steel. Sulfuric acid has theadvantages of lower cost, however, and can be processed in lead-linedtanks with lead steam coils and other lead-sheathed accessories, whereashydrochloric acid requires more expensive linings and accessories.

A typical pickling solution for cast iron makes use of one pint ofsulfuric and one pint of hydrochloric acid to one gallon of water,whereas for iron and steel, typical solutions are one gallon ofhydrochloric acid to one gallon of water, or one-half pint of sulfuricacid to one gallon of water. Having made the choice of an appropriatesolution for the article being pickled, it is important that theconcentration of acid be carefully maintained at its proper level in thecourse of pickling operations to prevent pitting and other adverseeffects. It is essential, therefore, that this concentration becontinuously monitored.

Conventionally, the acid concentration in a pickling bath is measuredinferentially either by measuring the percentage of iron and density, orby measuring the conductivity of iron and density. Techniques of thistype are more difficult to use in conjunction with closed-loop controlsystems for automatically adjusting acid concentration to correct fordeviations in acid concentration from a prescribed value. In order tooperate such a system, one must have available a control signal which isa function solely of the acid content of the pickling solution and noother variable, otherwise proper correction cannot be effected.

Accordingly, it is the main object of this invention to provide animproved technique and apparatus for determining the acid content ofpickling baths by generating a measurable substance in a quantity whichdepends solely on the acid concentration.

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More specifically, it is an object of the invention to provide ananalyzer for monitoring the acid concentration of a pickling bath,wherein the pickling bath solution is caused to react with a reagentconstituted by a halide and a halate salt to quantitatively liberate ahalogen in relation to the acid concentration, the halogen beingamperometrically measured to provide an output reading which reflectssolely the acid concentration.

A significant advantage of the analyzer as described above is that theoutput of the amperometric device may be used as a control signal tooperate a closed-loop system for varying the concentration of thepickling solution by adding acid or water in order to increase or reducethe concentration to an extent compensating for deviations in'acidcontent from a prescribed level.

Yet another object of the invention is to provide an acid contentanalyzer which is of relatively low cost and which is reliable andefficient in operation.

Briefly stated, in one embodiment of the invention, these objects areaccomplished in a system wherein a sample of an acid solution to betested is mixed with a reagent constituted by an aqueous solution of aniodide and iodate salt to liberate iodine as a direct function of acidconcentration, the iodine then being diluted with water and being fed toan amperometric device to produce an output current which is an analogof the iodine concentration of acid in the solution, the current actingto control a closed-loop system which varies the feed of water and acidinto the solution. to maintain a desired acid level therein.

For a better understanding of the invention, as well as other objectsand further features thereof, reference is made to the followingdetailed description to be read in conjunction with the accompanyingdrawing, wherein:

FIG. 1 is a graph explanatory of the chemical reaction between thepickling solution and the reagent;

FIG. 2 is a schematic diagram of the system in accordance with theinvention for measuring the concentration of'a pickling solution; and

FIG. 3 is a schematic diagram of a system for measuring theconcentration of soluble copper.

BASIC PRINCIPLES Amperometric instruments are currently available formeasuring halogens such as chlorine, bromine and iodine, in parts permillion, with a high degree of accuracy. An example of such instrumentsis the Anachlor, manufactured by the Fischer & Porter Co., ofWarminster, Pa., wherein a current flow is generated between theelectrodes of a cell proportional to the quantity of halogen in thesample.

In accordance with the present invention, the concentration of acid in apickling solution containing hydrochloric or sulfuric acid or both, ismeasured by reacting the pickling solution with a reagent constituted bya halide and a halate salt which liberates a halogen quantitatively inproportion to the acid concentration, the amount of halogen beingmeasured in a suitable amperometric instrument to produce a readingwhich is an index to the acid concentration. By halogen is meant any ofthe five elements, fluorine, chlorine, bromine, iodine and astatineforming part of Group VILA of the Periodic Table.

The reagent used in one embodiment of the system is an aqueous solutionof an iodide and iodate salt. Hydrochloric acid reacts with this reagentaccording to the following formula:

Sulfuric acid reacts with the reagent according to the followingformula:

Tests have shown that the reaction of pickling acid and the reagentproceeds rapidly and quantitatively. To demonstrate this reaction, knownamounts of hydrochloric acid were reacted with measured quantities ofiodide and iodate salts. The acid was varied from about to 15% ofpickling bath HCl. The iodine liberated was titrated by thewell-established starch-iodine method.

FIG. 1 shows a plot of the percentage of HCl added versus that found interms of equivalents of iodine. The iodine was rapidly andquantitatively produced if the theoretical amount of iodate and fourtimes the theoretical amount of iodide were present. A pickling bath isalso likely to contain significant amounts of dissolved iron. However, Ihave found that in amounts up to dissolved iron does not interfere withthe reagent-acid reaction and is not a material source or error. Thedissolved iron would normally precipitate from solution by reason of thefact that the acid is neutralized when reacted with the iodine andiodate salts. To prevent the precipitate from mechanically blocking thereactor and tubing, a third ingredient in the form of a chelating agentis added to the solution containing the iodide and iodate salts. Thetrisodium salt of EDTA (ethylene-diaminetetra-acetic acid) has beenfound to be suitable for keeping the iron in solution withoutinterfering with the primary reaction.

By the use of reagent-liberating iodine, it is also possible tocontinuously monitor soluble copper, ferric iron, hexavalent chromiumand manganese and solutions of hydrogen peroxide. In all cases, it isessential that the liberated iodine be diluted so that it is measurablewithin the operating range of the amperometric instrument used.

SYSTEM FOR MEASURING PICKLING ACID In the system shown in FIG. 2, use ismade of the Anachlor instrument, mentioned above, as an iodometricdevice for the measurement of the quantity of iodine liberated. If thepickling bath is sampled at full strength, the acid would then liberateiodine in amounts that are at least one thousand times the maximum rangeof this instrument (assuming a concentration of HCl at about 10% and H80 at about 30%). Hence it is necessary to dilute the sample to a levelcompatible with the range of the iodometric instrument used.

It will be appreciated that the degree of dilution disclosed herein isspecifically with reference to the Anachlor, but since the invention isoperable with other iodine-measuring amperometric instruments, inpractice the degree of dilution will be accommodated to the actualinstrument employed. In any event, dilution is desirable, for otherwisethe cost of the mixed iodideiodate reagent required to react completelywith the acid, would 'be prohibitive.

Referring now to FIG. 2, a sample solution derived from a pickling bathPB is first coarsely filtered and pumped by way of pump 1 and filter 2.A small portion of the output of pump 1 (about 10 cc./min.) is allowedto bleed through a valve 3 into a small container 4 that is open to theatmosphere. Container 4 is equipped with a constant overflow port 4A todrain. The remainder of the sample in the output of pump 1 is returnedto pickling bath PB.

The sample of pickling acid in the open container 4 is drawn, by meansof a micro-flow pump 5 through a heat exchanger 6 into a reactionchamber 7 wherein the sample is interacted with a reagent to liberateiodine. The reagent is drawn by a micro-flow pump 9 from a reagentreservoir 8 and fed into the reaction chamber 7. A typical operationmakes use of a sample acid rate of 0.02 cc./min. with a concurrentreagent rate of 0.7 cc./min.

The iodine generated in the reaction chamber 7 is fed into a line 10containing dilution water which is preferably of drinking quality. Thiswater first passes through an orifice 11 which is adapted to restrictits flow to about 300 cc./min. This rate is held constant by adifferential pressure regulator 12. After leaving the regulator, thedilution water is conducted through a dechlorinator 13 and then throughheat exchanger 6 where it serves to cool the pickling acid samplepassing through the exchanger.

From the exchanger, the water goes into the line 10 where it picks upthe iodine from the reaction chamber. The water and iodine in line 10 isthen blended by a mixer 14, and the diluted iodine is fed into theanalyzer which in practice may be an Anachlor. The degree of dilution issuch as to bring the diluted iodine within the operating range of theanalyzer used. All lines in the system are made as short as possible toreduce lag time.

The output of analyzer 15 may be read on a microammeter directlycalibrated in terms of acid concentration, and this output may berecorded to provide a running record of the varying acid concentration.To efiect automatic correction of the pickling bath concentration, theoutput of analyzer 15 is applied as a control signal to a closed servoloop system 16 of conventional design, where the control signal iscompared with a reference signal representing the desired acidconcentration to produce an error signal whose magnitude and sensedepend on the extent and direction of the departure of the acidconcentration from the reference value. This error .signal is used toadjust valves 17 and 18 feeding acid and water into the pickling bath toa degree returning the solution to its prescribed concentration.

SYSTEM FOR MEASURING SOLUBLE COPPER In order continuously to measuresoluble copper in aqueous solution, an iodide salt (i.e., potassiumiodide) is added to the solution to liberate iodine in amountsproportional to the copper present, assuming the absence of otheroxidizing agents. The reaction is as follows:

Hence as a continuous stream containing copper is treated with an iodidesalt, the iodine liberated according to the above reaction may then bemeasured by an Anachlor or a similar amperometric cell. It has beenfound that if one grain of potassium iodide is added to each 0.6milligram of copper sampled, the iodine is instantaneously liberated inproportion to the copper present.

As shown in FIG. 3, the process stream is sampled by a micro-pump 19 ata low rate of about 20 cc./min., in the interest of economy, for the KIrequirement is high. The sampling rate depends on the range over whichthe copper may be expected to vary. Ideally about 0.4 milligram ofsoluble copper should be sampled per minute. The output of micro-pump 19is fed to a reaction chamber 20 to which is also fed a KI solution by amicropump 21 at 1 cc./min. from reservoir 22. The iodine liberated inthe reaction chamber is diluted with water which is fed at cc./min.through a dechlorinator 23 to the analyzer 24, this being necessarysince the Anachlor is also responsive to changes in chlorine residual.

The invention is not limited to use with reagents constituted by iodideand iodate salt to produce a reaction in which iodine is liberated. Theinvention encompasses reagents constituted by a halide and halate saltto produce a reaction in which liberated halogen is measured. Thus insome situations, it is preferred to use a bromide and bromate salt inthe reagent and to measure the liberated bromine.

While there have been shown and described preferred embodiments ofcontinuous analyzers for acid content and monitoring system inaccordance with the invention, it will be appreciated that many changesand modifica tions may be made therein without, however, departing fromthe essential spirit of the invention as defined in the annexed claims.

What I claim is:

1. A system for solely determining the concentration of a particularsoluble constituent in an aqueous solution having other constituents,which particular constituent is capable of liberating halogen when thesolution is reacted with a reagent including halide and a halate salt,said system comprising an amperometric device responsive to halogens togenerate a current in proportion to the concentration of the appliedhalogen, said device having a predetermined maximum range, means toderive a sample of the solution to be tested, means to react the samplewith said reagent to liberate a halogen, means to dilute the halogenwith water to produce a dilute sample, whose degree of dilution producesa halogen concentration within the range of said device, and means toapply said dilute sample to said device to determine the concentrationof said constituent in said aqueous solution.

2. A system as set forth in claim 1, wherein said reagent includes aniodide salt.

3. A system as set forth in claim 1, wherein said reagent includes abromate salt.

4. A system as set forth in claim 1, wherein said constituent ishydrochloric acid.

5. A system as set forth in claim 1, wherein said constituent issulfuric acid.

6. A system as set forth in claim 1, wherein said constituent is solublecopper.

7. A system for determining the concentration of hydrochloric andsulfuric acid in a pickling solution, said system comprising:

(a) a reaction chamber,

(b) means to feed into said chamber a sample of said pickling solution,

(c) means to feed into said chamber a reagent constituted by a halideand a halate salt to produce a reaction in which a halogen is liberatedin proportion to the concentration of acid in said solution sample,

((1) means to draw said halogen from said chamber and to dilute it withwater drawn from a source to produce a halogen sample, whoseconcentration lies 6 within the range of the amperometric device towhich it is fed,

(e) an amperometric device responsive to halogens to produce an outputcurrent as a function thereof, said device having a predetermined range,and

(f) means to feed said halogen sample to said device to provide ameasurement of said acid concentration.

8. A system as set forth in claim 7, wherein said reagent is constitutedby iodide and iodate salt.

9. A system as set forth in claim 7, wherein said reagent is constitutedby bromide and bromate salt.

10. A system as set forth in claim 7, further including a closed-loopcontrol system responsive to said output current to produce an errorsignal which depends on the deviation of the measured acid concentrationfrom a desired value, said control system acting to adjust theconcentration of said pickling solution to said desired value.

11. A system as set forth in claim 7, further including a heat exchangerand means to feed said solution sample into said reaction chamberthrough said exchanger in heat exchange relationship with the waterdrawn from said source to cool said solution sample.

12. A system as set forth in claim 7, further including a mixer to blendthe halogen with the Water fed into said analyzer.

References Cited UNITED STATES PATENTS 1,036,121 8/1912 Jones et al.23216 1,944,738 1/ 1934 Grebe. 2,834,654 5/ 1958 Murayama.

MORRIS O. WOLK, Primary Examiner R. E. SERWIN, Assistant Examiner US.Cl. X.R. 23-215, 230

